Renault signed the Spaniard for this year only, despite the fact that Red Bull could pull him back into their family to replace Daniel Ricciardo.
Team advisor Prost told Italy's Autosprint: "We knew taking Sainz would be a risk. But it is important to take this kind of risk to advance the team.
"It was a positive decision for the team and for Nico [Hulkenberg], because you can see how it has raised [Hulkenberg's] level even further."
Renault are fifth in the constructor standings after the first three races of 2018.